gpm: sets D equal to DESTDIR instead of ROOT in do_install

This corrects a gpm install failure due to ROOT being used instead of
DESTDIR, resulting in DESTDIR being undefined.

Upstream-Status: Inappropriate (distribution/packaging fix)

Signed-off-by: Stephen Arnold <stephen.arnold42@gmail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>